<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=no" />
    <title>Document</title>
  
  </head>
  <body>
    <div id="app">
      firstName: <input type="text" v-model="firstName">
      lastName: <input type="text" v-model="lastName">
      =>
      {{fullName}}
    </div>
    <script src="./vue.global.js"></script>
    <script>
      const { createApp } = Vue;
      const app = createApp({
        data() {
          return {
            firstName: '',
            lastName: '',
            fullName: ''
          };
        },
        watch: {
          firstName(val){
            this.fullName = val + '.' + this.lastName;
          },
          lastName(val) {
            this.fullName = this.firstName + '.' + val;
          }
        }
      });
      app.mount("#app");
    </script>
  </body>
</html>
